While, as will be understood from the following description, the present invention was developed for efficiently and effectively measuring long aircraft metal parts in a way that compensates for the thermal expansion and contraction of such parts, this invention may also find utility in other environments.
Aircraft endure great stresses and forces during their expected operational life, which is relatively long-more than twenty (20) years. When inconsistently dimensioned parts are included in an aircraft assembly, the ability of the aircraft to adhere to its design criteria and resist the stresses and forces applied to the aircraft may not be achieved. In order to avoid the inclusion of inconsistently dimensioned parts in an aircraft assembly, prior to assembly parts are measured to determine if they are sufficiently accurate.
Many structurally important aircraft parts are long-over twenty-four inches (24"). Currently there are three common methods of measuring long metal (e.g., aluminum) aircraft parts--using a standard (25 ft.) hand-held tape measure; laying a full-scale drawing on a part and comparing the drawing dimensions with the part dimensions; and transporting the part to a Coordinate Measuring Machine (CMM) location. Each method has disadvantages for in-process (i.e., during manufacturing and assembly) measuring.
The use of a standard hand-held tape measure does not provide sufficiently accurate, repeatable measurements. Drawings overlaid on a part may move during a dimension check. Further, any deformity of the drawing medium will make it difficult, if not impossible, to make an accurate dimension check. CMMs are not suited for real-time in-process production measuring for several reasons--the measuring process is lengthy, up to eleven hours in some instances; CMMs require highly trained operators; and CMMs are expensive, which limits the number of CMMs that can be provided to a production facility. Further, none of the foregoing methods provide temperature compensation. In this regard, all metal parts are subject to thermal expansion and contraction. While the amount of thermal expansion of small parts (or the short dimensions of elongate parts) is small and can usually be ignored, the longitudinal thermal expansion of long metal parts can be significant and must be taken into consideration if precisely assembled aircraft structures are to be produced.
Prior attempts to solve some of the disadvantages of the methods described above have involved using an optical encoder in combination with rack sections linearly arrayed on and attached to a table top. While very accurate over relatively short distances, this approach becomes increasingly less accurate with length due to the difficulty of maintaining the rack sections in precise linear alignment. Further, this approach does not provide temperature compensation.
In the past, temperature-compensated part measurement has involved enclosing the entire measurement system (i.e., an entire CMM) in a temperature-controlled room or building. While this approach is effective, it is also expensive and time consuming since parts must remain in the temperature-controlled environment for several hours (sometimes twelve to fourteen hours) before testing so that they can stabilize to the ambient temperature of the environment. Thus, this method is ineffective for use as an in-process production measuring device. If parts are only periodically measured using a CMM in a temperature-controlled environment, several incorrectly sized parts may be produced before a negative feedback from the CMM occurs. More importantly, in aircraft assembly, an unchecked, out-of-tolerance part may reach an aircraft assembly point and cause a "line" stoppage, shutting down the assembly of the related section of the aircraft. More specifically, present-day production philosophy is just-in-time parts manufacturing. As a result, there are very few parts, and ideally only one part, available at the point of assembly. A part that does not fit can shut down assembly, not only of the aircraft being assembled but also aircraft further down the assembly line. As a result, construction time is wasted producing bad parts.
The present invention is directed to overcoming the foregoing and other disadvantages. More specifically, the present invention is directed to providing a method and apparatus suitable for in-process production measuring of long parts that exhibit thermal expansion without the use of highly trained operators and expensive temperature-controlled environments.